1. Field of the Invention
The present invention relates to an intermediate transfer device, an image forming apparatus using the same and a secondary transfer method.
2. Discussion of the Background
Among color image forming apparatuses employing electrophotography, for example, tandem type image forming apparatuses widely employ a system in which an image formed on an intermediate transfer belt by primary transfer is secondarily transferred to a recording medium such as paper. When a transfer roller is used as a secondary transfer device that transfers a toner image formed on an intermediate transfer belt to a recording medium by such secondary transfer, the transfer speed of the intermediate transfer belt varies due to the shock at the time when the recording medium enters into the secondary transfer portion. This distorts images, which is referred to as shock jitter.
A technology that is known to prevent this shock jitter describes an image forming apparatus having a toner image bearing member that bears a toner image, a pressure transfer body arranged in the vicinity of the toner image bearing member, a transfer medium thickness detection device to detect the thickness of a transfer medium, and a gap adjustment device. The pressure transfer body presses the transfer medium entering between the toner image bearing member and the pressure transfer body to the toner image bearing member to transfer (and attach) a toner image thereon to the transfer medium. The gap adjustment device automatically changes the gap between the toner image bearing member and the pressure transfer body according to the detection information from the transfer medium thickness device.
In addition, another technology to adjust the gap describes an image forming apparatus having an image bearing member that rotates and bears an image, a transfer member, a recording medium transfer device, and a gap formation device. The transfer member rotates in contact with the image bearing member and transfers an image formed on the surface of the image bearing member to a recording medium. The recording medium transfers the recording medium to the contact position between the image bearing member and the transfer member. The gap formation device forms a gap at the contact position just before the recording medium enters into the contact position.
The technology described above first detects the thickness of a transfer medium and adjusts the gap between the secondary transfer portions to securely relax the shock occurring when the transfer medium enters between a toner image bearing member and a pressure transfer body or is released from therebetween regardless of the thickness of the transfer medium. However, the shock jitter is affected by stiffness and the from of the front end of a transfer medium which vary depending on the kind of the recording medium. Therefore, the shock jitters are not completely prevented by simply adjusting the gap based on the information on the thickness of the recording medium. In addition, the technology secondarily described above sets the timing for forming the gap, but is not sufficient to completely prevent the occurrence of shock jitters.